The first is that he has to deal combat damage, which means he likely has to use his own ability to grant himself unblockable, costing a bit of mana
Best of all, Brinelin has Partner, which means you're not limited to having a mono-blue deck when playing with it as your commander
Avoid mass-produced variants, high-pop cards without character significance, and pure speculation plays
Weakness: Fighting (x2) Resistance: Metal (-20) Retreat: 1 Rating: 5|5 - Format defining Search: 4|5 - Ultra Ball and Nest Ball Value: 5|5 - This effect normally requires two Stage 1 Pokemon Compare this to something like Aqua Patch: Not only can you search it with Nest Ball and Ultra Ball, but it offers twice as much value per deck slot as Aqua Patch
Consider the fact that every game has its own die-hard fans who hang on to every character and lore tidbit, and things get even tougher
That said, three cards have returned from being Forbidden to now being newly Limited, and the first one was one everyone saw coming a mile away, being Metamorphosis